ABO Incompatibility and Hematopoietic Stem Cell Transplantation Outcomes.

Introduction: The increased risk of hemolytic reactions and erythrocyte recovery delay in ABO incompatible hematopoietic stem cell transplantation (HSCT) are well established. Effects of ABO incompatibility on other transplantation outcomes are evaluated in this study. Subjects and Methods: We prospectively followed 501 patients undergoing allogeneic stem cell transplantation regarding their ABO compatibility groups for a median time of 34.7 months. Patients were studied in minor, major and bidirectional mismatched and matched groups. Results: Mean survival time (OS) was lower in minor mismatched group (p-value= 0.017). Minor and bidirectional mismatched groups received significantly more packed cell units than matched group (p-value < 0.0001 and p-value =0.002, respectively).Mean number of platelet unit infusion was significantly more in major mismatched recipients than matched group (p- value=0.031). Death rate was much more than expected in minor mismatched group. Two cases of PRCA (pure red cell aplasia) were found in major mismatched group. No statistically significant difference was found in the incidence of acute GVHD, chronic GVHD, time to neutrophil recovery, relapse- free survival, non-relapse mortality and relapse rate among groups. Conclusion: In order to prevent complications of ABO-incompatible SCT such as decrease in OS and the need for more transfusions, choosing ABO-compatible donors would improve transplantation outcomes.


INTRODUCTION
In contrast to solid organ transplantation, HLA matching is critical in allogeneic hematopoietic stem cell transplantation (HSCT) and ABO incompatibility is not considered a barrier, but it seems that erythrocyte recovery delay and hemolytic complications maybe a problem by the presence of recipient antibodies against donor ABO antigens. The increased risk of hemolytic reactions in ABO incompatible HSCT is well understood. 1 Theeffect of ABO incompatibility on transplantation such as overall survival (OS), graft-versus-host disease (GVHD), and relapse is still controversial. 2,8 ABO incompatibility has three features: major incompatibility, that happens when the recipient with O blood group receives graft from A/B/AB donor, minor incompatibility which occurs when the donor with anti A/B antibodies donates stem cells to a patient with A/B or AB blood group and bidirectional incompatibility is defined when both donor and recipient have anti ABO antibodies. In this study, we evaluated graft outcomes regarding ABO

SUBJECTS AND METHODS
We prospectively followed the patients (n=501) undergoing allogeneic stem cell transplantation between 2010 and 2012 in our center for a median time of 34.7 months. Patients' characteristics are shown in Table1. Glucksberg system by presentation and staging of gastrointestinal, liver and skin GVHD at least seven days after transplantation. 9 Absolute neutrophil count (ANC) more than 0.5 *109/L for 3 consequent days was considered as neutrophil recovery and platelet engraftment was determined by platelet count of greater than 20*109/L for three consequent days without any supplementary platelet. Chimerism was assessed on days +15, +30, +60 and +90. Relapse and secondary graft failure were identified by clinical and /or hematologic recurrence or chimerism decline. Death due to treatment except relapse was defined as nonerelapse mortality (NRM). Dates of relapse and death were recorded to identify relapse-free survival (RFS) and overall survival (OS). All participants signed the informed consent forms.

Statistical Analysis
Statistical analysis was performed using SPSS version 22.0. The incidences of death, relapse, acute and chronic GVHD were compared in each ABO blood group incompatibility using cross-tab tables with likelihood-ratio χ2 statistics. The means of recipient's and donor's age, platelet and WBC engraftment, packed cell and platelet infusion were compared using ANOVA and Kruskal-wallis with post-hoc statistics. Overall survival and relapse-free survival were analyzed by the Kaplan-Meier method, and the Breslow test was used to examine significant differences among blood group compatibilities. Factors that significantly affected survival and relapse-free survival were evaluated by the Cox proportional hazards multivariate model. Logistic regression multivariate analysis was used to determine significant effects of variables on incidence of mortality, relapse, and non-relapse mortality, acute and chronic GVHD as outcomes. 10, 12

RESULTS
According to ABO compatibility of donors and recipients, four groups were distinguished: match, major mismatch, minor mismatch and bidirectional ABO mismatch. Recipient's gender, gender mismatch, conditioning regimen, HLA matching, primary disease, stem cell source, receiving ATG(anti-thymocyte globulin) in conditioning regimen, mononuclear cells, CD34 and CD3 cell doses were almost equally distributed in these four groups. Only recipient's age was different among groups (p-value= 0.034). Table 1 shows univariate analysis of patients' characteristics. Univariate analysis of transplantation outcomes regarding BO compatibility groups showed no significant difference inaGVHD, cGVHD, time to neutrophil recovery, NRM and relapse rate in all groups. Chimerism was different among groups, but it was not significant (p-value=0.078).Mean days of platelet engraftment, units of packed cell transfusion, units of platelet infusion and death incidence rate were statistically different ( Table 2). Minor and bidirectional mismatched groups received significantly more packed cells than matched group (p-value<0.0001 and p-value =0.002, respectively). Although major mismatched patients received more packed cells than matched group, the difference was not significant (p value =0.06).The Mean number of platelet units transfused was significantly more in major mismatched recipients than matched group(p-value=0.031). Death rate was less than expected in matched group and it was much more than expected in minor mismatched group(Table2). Totally, overall survival was 39.8 months (95% CI: 38.1 -41.5). In univariate analysis, mean survival time (OS) was statistically different among groups (p value= 0.017) and the worst result was found in minor mismatched one .Relapse-free survival (RFS) was 45.3 (95% CI: 44.0 -46.7) in all patients and it was lower in minor mismatched group, but the difference was not statistically significant (p value=0.30) In multivariate analysis of overall survival, minor mismatch ABO incompatibility decreased survival (RR: 2.29, CI: 1.47-3.55, p value<0.0001). Other ABO incompatibilities did not affect survival (    Table 3 also includes Cox regression multivariate analysis results for RFS. ABO incompatibility was not correlated with RFS (Figure 1-B  Pure red cell aplasia (PRCA) occurred in two male patients (with ALL and SAA) in major mismatched group; one of whom had received cells from female and the other from male HLA -identical sibling donor.They received MA and non-MA regimen as conditioning, respectively.aGVHDof liver (grade II-III) and also cGVHD were presented in both patients.

DISCUSSION
Approximately one-third of bone marrow or peripheral blood stem cell transplantations are performed with ABO blood group incompatibility. 2,13 In this study, we evaluated the impact of ABO mismatch on outcomes such as OS, RFS, and NRM, time to engraftment, relapse and also GVHD. A decrease in OS was only observed in patients undergoing minor blood group mismatched HCT. We observed that a decrease in OS occurred only in patients with minor blood group mismatched transplantations. RFS was lower in minor mismatched grafts, but the difference was not statistically significant. Similar to our results, Ozkurt et al.'s and Logan et al.'s studies also reported a significantly shorter OS in recipients with minor ABO-mismatched grafts. 14,15 Stussi et al. observed an independent decrease in survival after bidirectional ABO-incompatible SCT, but it was not found in the minor or major ABO-incompatible groups. 2 Three hundred thirty-eight patients with ABOincompatible SCT were evaluated by Mielcarek et al., and there were no significant differences in survival and GVHD among the ABO-incompatible groups. 3 Some other studies have also reported no relationship between ABO groups and OS. 3,5,6,16 In a large retrospective study conducted in Japan, OS was significantly lower in major and minor mismatched groups than the AB0-identical group. 17 16 we observed no difference in relapse and NRM between ABO-pairs. But Kimura et al. showed higher NRM in the major and minor mismatched groups. Meanwhile, just like our results,they did not find any significant difference in rate of relapse. 16 In 2015, Biology of Blood and Marrow Transplantation Journal published an article reporting an increase in NRM of minor mismatched groups. 15 Some studies that ABO incompatibility may be associated with increased risk of GVHD. [17][18][19] In one report, minor ABO incompatibility was related with a higher risk of severe acute GVHD in comparison to other groups, 20 but in our study, aGVHD and cGVHD were not statistically correlated with ABO compatibility. Mielcarek et al. and also Kim et al. have also reported similar results. 3,6 Pure red cell aplasia (PRCA) occurred in two male patients (with ALL and SAA) of our major mismatched group. They received transplantation from female and male HLA -identical sibling donors, respectively. PRCA has also been reported after major ABO-incompatible stem cell transplantation in other studies. 2 Considering relatively diverse results on this topic in the literature, a study with a larger sample size and also a meta-analysis could probably help achieve more accurate results.

CONCLUSION
Since unfavorable outcomes such as decrease in OS and the need for more transfusions are statistically significant in ABO incompatible SCT and also complicationsuch as PRCA is observed in these patients, we suggest that in clinical practice, if a given patient has several suitable donors, the one with a compatible ABO blood group would improve outcomes of the transplantation. Considering relatively diverse results on this topic in the literature, a study with a larger sample size and also a meta-analysis could probably help achieve more accurate results.